Due to depleting world energy resources and global warming, there is a drive to improve energy efficiency of new and existing commercial and residential buildings. One of the strategies is to improve thermal insulation around the buildings. Currently, the building industry uses several different forms of insulation materials, for example, glass fibers and mineral fibers. However, glass and mineral fibers exhibit high thermal conductivity in the range of 0.03-0.04 W/m·K. In comparison, aerogels exhibit thermal conductivity in the range of 0.008-0.012 W/m·K, but aerogels are very fragile and lack the mechanical strength needed for thermal insulation for building applications.
Apart from fibrous insulation, certain types of polymeric foams are commonly used for insulation applications that exhibit thermal conductivity in between those of glass fibers and aerogel materials. Only foams that are blown from low thermal conductivity blowing agents and result in a predominantly closed cell structures, with significant fraction of the blowing agent trapped within the closed cells, can exhibit low thermal conductivity and high insulating values. Commercial foams with high insulation value are blown from low temperature boiling liquids such as hydrocarbons and hydro fluorocarbons (HFCS) which exhibit a gas phase thermal conductivity in the range of 0.008-0.015 W/m·K. Therefore, the foams that result from such blowing agents can exhibit thermal conductivity in the range 0.018-0.030 W/m·K. However, some of the hydrocarbons and hydro fluorocarbons (HFCs) are being phased out due to theft ozone depletion potential (ODP) and global warming potential (GWP).
Furthermore, closed-cell foams derived from polystyrene and polyurethane that can have a thermal conductivity of less than 0.03 W/m·K are highly flammable and thus have limited application as building insulation material even with the addition of flame retardants. Foams derived from polyisocyanurates exhibit better flame resistance than polystyrene and polyurethane, and phenolic foams exhibit even better flame resistance than polyisocyanurate foams. However, phenolic foams use a phenol based monomer which is produced from a petroleum feedstock, a depleting non-renewable resource.
Tondi et al., Microscopy and Microanalysis, 15, 384-394, doi: 10.1017/S1431927609990444 disclose X-ray microtomography of tannin-based rigid foams of different bulk densities. Quantitative analysis of X-ray microtomography pictures showed negligible closed porosity for all foams, whereas helium pycnometry showed closed porosity in the range of 4.5-7% for all foams.
Hence, there is a need for low thermal conductivity and fire resistant polymeric foams from renewable sources having a closed-cell structure with trapped blowing agent preferably with low ODP and low GDP.